Airborne measurement of the temperature of the cosmic microwave background at 3.3 mm

THE ‘primaeval fireball1 model of the Universe’ predicts a cosmic microwave background radiation field which has the spectral character of a blackbody source. The radiation arises from an initially very dense, very hot Universe, and has been cooled to around 2.7 K through the cosmological expansion. Previous ground-based radiometric measurements2–6 at wavelengths as short as 3.3 mm are consistent with a 2.7 K blackbody, but most of these points are in the Rayleigh-Jeang tail of such a blackbody spectrum.